The present invention relates to a color conversion method that enables a color image generated for one type of output device to be output by a different device.
Color conversion becomes necessary when, for example, a color image generated in a computer for display on a cathode-ray-tube (CRT) monitor is printed on paper by an ink-jet color printer, or an electrophotographic color printer. The CRT employs additive color mixing of the three primary colors red, blue, and green. The color printer employs subtractive color mixing of inks (or toner) of the three primary colors cyan, magenta, and yellow, or the four colors cyan, magenta, yellow, and black. The CRT can generally reproduce a larger gamut of colors than the printer, so a color conversion or mapping process is required. Various methods have been proposed.
Japanese Kokai Patent Application 88589/1993 describes a method of projecting the CRT's color space onto the printer's color space along lines linking each color to the white point on the chromaticity diagram. For each such line, the ratio of the most saturated color reproducible by the printer to the most saturated color reproducible by the CRT is determined, and the entire line is contracted or expanded by that ratio.
In practice, these most saturated reproducible colors can be difficult to determine. This is particularly true of the printer, because of extraneous absorption by each color of ink. Essentially the only way to determine the full gamut of colors that a color printer can reproduce is by actual measurement of samples of all the colors the printer can print. The measurement results would then have to be stored in some systematic way and referred to in the color conversion process, but the amount of data to be measured, stored, and referred to would be impractically large.
Japanese Kokai Patent Application 34546/1996 describes a method that constructs a polygon in the chromaticity diagram for each lightness level. The vertices of the polygon represent the maximum reproducible saturations of hues corresponding to the primary pigments used by the printer, and to secondary combinations of these pigments. This polygon approximately outlines the gamut of colors that the printer can reproduce. Input colors disposed within the polygon are left unaltered. Input colors external to the polygon are mapped onto the intersection of an edge of the polygon with a line joining the input color to the white point in the chromaticity diagram.
Use of a polygonal approximation reduces the amount of data to be stored, although if a separate polygon is used for each lightness level, the amount of data is still large. Mapping all colors external to the polygon onto the edges of the polygon eliminates the need to determine the most saturated reproducible colors in the input system, although since a range of saturations is mapped onto a single point, there is a loss of saturation information. Considerable vividness may also be lost in this way, as described later.
A problem with both of these methods is that the white point on the chromaticity diagram of the CRT rarely coincides with the white point on the chromaticity diagram of the printer. The white point of a color printer is the paper white color: the color of the paper on which the image is printed, which virtually never matches the white point of the CRT. A further complication is that in a color printer, differing from a CRT, the chromaticity coordinates of the color black (either the color of the black ink, or the color black as reproduced by mixing cyan, magenta, and yellow) rarely match the chromaticity coordinates of the color white. As a result, when the above methods are applied, hues are altered, points that should be white, gray, or black become tinged with color, and the quality of the printed image is visibly degraded.